1. Field of the Invention
This invention relates to the treatment of coal and particularly relates to admixture of coal with additives to improve its ignition, reduce its spontaneous combustibility, reduce its sulfur emission, minimize its pick-up of moisture and control its flow properties in finely divided form. More particularly, the invention relates to treatment of coal to provide a uniform and easily handled fuel, in powdered or in briquette form, for use in small furnaces.
2. Review of the Prior Art
At the present time, petroleum imports account for more than 40% of United States usage. At any time, this supply could be curtailed or eliminated by political or military action. Even if such an emergency situation does not develop, liquid hydrocarbon fuels could become increasingly scarce in view of government policies that affect domestic exploration, development, and refining. As such scarcity develops, stationary furnaces and power plants, including domestic and commercial furnaces for space heating and hot water, will tend to be fueled with solid fuels, particularly coals and other bio-mass such as wood products.
However, there are some important obstacles that will have to be overcome, such as the distribution of coal to households, the inconveniences encountered in burning coal as compared to gas and oil burning, and the environmental impact created by sulfur dioxide emission and by particulate emissions. Indeed, in preparing for an emergency situation, it is important to design a coal-based material and a coal-burning system that can be promptly put into use or which at least can be readily promoted as a substitute for liquid and gaseous fuels in domestic and small commercial applications. Such a coal-based material could be developed by suitable "refining" and blending of various coals with each other and with additives to make a composite material which is suitable for these special applications.
An important problem that can be expected to plague small domestic usage of coal is spontaneous combustion which is caused by slow oxidation of the coal without adequate opportunity for dissipating the resultant heat when the coal is being stored in piles and bins. In a given weight of coal, the rate of oxidation increases with the increase in exposed surface area, so that spontaneous combustion is more apt to occur in coal piles having an excessive amount of fines.
U.S. Pat. No. 1,545,620 discloses pulverized coal admixed with a hydrocarbon oil and limestone. U.S. Pat. No. 3,961,914 discloses a method for making coal resistant to spontaneous combustion by coating it with silicon dioxide. U.S. Pat. No. 3,985,516 discloses the coating of coal with a heavy liquid hydrocarbon material to prevent spontaneous combustion. U.S. Pat. No. 3,867,109 discloses a treatment process comprising adding heavy oils, having a specific range of viscosity, to powdered coal, heating the mixture at 100.degree. F., and flashing it to 100.degree. F. in order to improve its bunkerability or storage properties. U.S. Pat. No. 3,288,576 teaches the admixing of coal with an antioxidant to inhibit oxidation of the coal. U.S. Pat. No. 3,723,079 describes a process for stabilizing dried lignitic and sub-bituminous coal against spontaneous combustion which comprises treating the dried coal at about 175.degree.-225.degree. C. with 0.5-8% oxygen by weight and rehydrating the oxygen-treated coal with 1.5-6% by weight of water.
Other problems that have caused serious difficulties are the corrosion of furnaces and pollution of air by the sulfur in coals. Coals from the eastern United States particularly tend to be high in sulfur.
Efforts to remove or minimize the effects of sulfur have included the addition of sodium carbonate, as in U.S. Pat. No. 1,007,153, the addition of calcium sulfate or calcium phosphate as in U.S. Pat. No. 2,247,415, the addition of limestone or dolomite as in U.S. Pat. No. 3,640,016, mixing coal with water and heating the mixture in a non-oxidizing atmosphere at a temperature near the critical temperature of water and at a pressure high enough to maintain the water in a liquid state as in U.S. Pat. No. 3,660,054, admixing coal with manganous-amine complexes to reduce sulfur oxide and nitrogen oxides as in U.S. Pat. No. 3,443,916 and reacting coal with hydrogen to form hydrogen sulfide as in U.S. Pat. No. 3,909,212.
An additional problem that can be serious in any furnace is the formation of a slag that sticks to the furnace walls or bottom or the formation of an ash that will not agglomerate but is readily carried away with the combustion gases as fine particles into the atmosphere. The addition of sand and limestone have been proposed for correcting this condition, as in U.S. Pat. No. 1,150,839. The inhibition of slag formation is taught in U.S. Pat. No. 3,004,836 which discloses the addition of magnesium oxide and phosphate rock to certain coals.
Another pertinent factor in any proposed usage of coal for small furnaces is the variability of coal as to calorific value, chemical content, specific gravity, hardness, and size stability. Chemical content is commonly reported as a proximate analysis which is broken down into the following four items which must add up to 100%: moisture, volatile matter, fixed carbon, and ash. Chemical content when reported as an ultimate analysis is the summed percentages of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash.
Specific gravity of pure coal varies from 1.25 to 1.70, generally increasing with rank or content of fixed carbon. Hardness or firmness depends upon the composition and location of the coal bed and may affect the grindability of coals. High volatile coals are more firm than low volatile coals. Size stability is the ability of coal to withstand breakage and further attrition, as would occur in handling, shipping, and delivery to the burner in the small furnace. This property is important because although a finely divided coal is desirable for proper handling and efficient burning, extreme dustiness is undesirable and may even be dangerous.
Ignition and combustion can also be problems when coal is employed in automatic-starting small furnaces, particularly during cold weather and when the furnaces are also cold. As a combustion improver, iron, manganese, and copper admixed with compounds of lead, cobalt, nickel, chronium, antimony, tin, and vanadium have been proposed in U.S. Pat. No. 3,348,932 to improve the burning properties of coals.
U.S. Pat. No. 3,332,755 also discloses fuel compositions of petroleum, coal, and coke which contain additives of aluminum, magnesium, and manganese. The additive mixture also contains a surfactant, a diluting oil (petroleum distillate), and glycol.
Coal is classified as to rank or the percentage of total carbon that occurs in complex, condensed, ring structures. The carbon content of coal supplies most of its heating value and is commonly reported as fixed carbon, the combustible residue left after driving off the volatile matter, although this material is not all carbon. In inverse proportion to rank is the content of volatile matter. Anthracite has the highest rank or proportion of fixed carbon, followed by bituminous, sub-bituminous, and lignite. Bituminous coal is classified as low-volatile bituminous, medium-volatile bituminous, and high-volatile bituminous.
Because such variations in coal as to chemical and physical properties can create serious problems in operating small furnaces which must be capable of operation without expert attendance, it is critically important for this invention that the various ranks and types of coals, after suitable size reduction, be blended to obtain a blend or average composition that will give a uniform heat content of about 12,000-14,000 Btu per pound, reproducible flow properties, optimum storage properties, and minimal slag, agglomeration, fly-ash content, and sulfur content.